Linearly Solvable Stochastic Control Lyapunov Functions

This paper presents a new method for synthesizing stochastic control Lyapunov functions for a class of nonlinear stochastic control systems. The technique relies on a transformation of the classical nonlinear Hamilton-Jacobi-Bellman partial differential equation to a linear partial differential equation for a class of problems with a particular constraint on the stochastic forcing. This linear partial differential equation can then be relaxed to a linear differential inclusion, allowing for relaxed solutions to be generated using sum of squares programming. The resulting relaxed solutions are in fact viscosity super/subsolutions, and by the maximum principle are pointwise upper and lower bounds to the underlying value function, even for coarse polynomial approximations. Furthermore, the pointwise upper bound is shown to be a stochastic control Lyapunov function, yielding a method for generating nonlinear controllers with pointwise bounded distance from the optimal cost when using the optimal controller. These approximate solutions may be computed with non-increasing error via a hierarchy of semidefinite optimization problems. Finally, this paper develops a-priori bounds on trajectory suboptimality when using these approximate value functions, as well as demonstrates that these methods, and bounds, can be applied to a more general class of nonlinear systems not obeying the constraint on stochastic forcing. Simulated examples illustrate the methodology.Comment: Published in SIAM Journal of Control and Optimizatio

Institutions and the Volatility Curse

This paper revisits the resource curse paradox and studies the impact of resource rents and their volatility on economic growth under varying institutional quality. Using five-year non-overlapping observations between 1970 and 2005 for 112 countries, we find that while resource rents enhance real output per capita, their volatility exerts a negative impact on economic growth. Therefore, we argue that volatility, rather than abundance per se, drives the resource curse. However, we also find that higher institutional quality can help offset some of the negative volatility effects of resource rents. Therefore, resource abundance can be a blessing provided that growth and welfare enhancing policies and institutions are adopted

Green Gasification Technology for Wet Biomass

The world now is facing two energy related threats which are lack of sustainable, secure and affordable energy supplies and the environmental damage acquired in producing and consuming ever-increasing amount of energy. In the first decade of the twenty-first century, increasing energy prices reminds us that an affordable energy plays an important role in economic growth and human development. To overcome the abovementioned problem, we cannot continue much longer to consume finite reserves of fossil fuels, the use of which contributes to global warming. Preferably, the world should move towards more sustainable energy sources such as wind energy, solar energy and biomass. However, the abovementioned challenges may not be met solely by introduction of sustainable energy forms. We also need to use energy more efficiently. Developing and introducing more efficient energy conversion technologies is therefore important, for fossil fuels as well as renewable fuels. This assignment addresses the question how biomass may be used more efficiently and economically than it is being used today. Wider use of biomass, a clean and renewable feedstock may extend the lifetime of our fossil fuels resources and alleviate global warming problems. Another advantage of using of biomass as a source of energy is to make developed countries less interdependent on oil-exporting countries, and thereby reduce political tension. Furthermore, the economies of agricultural regions growing energy crops benefit as new jobs are created.Keywords: energy, gasification, sustainable, wet biomas

A framework and tool for supply network strategy operationalisation

This research aims to develop a framework to capture the interplay of network strategic decisions, and to understand how firms operating within a supply network interact with each other. The research involves an in-depth longitudinal case study in one supply network consisting of 20 firms. Data was systematically collected through the use of a Supply Network Analysis Process (SNAP) methodology. Interviews and secondary data were collected for further data validation and triangulation. This paper describes the above case study in detail, explains the SNAP methodology and framework development, and discusses the implications of the research both in academia and industry

A Comment on the Implementation of the Ziggurat Method

We show that the short period of the uniform random number generator in the published implementation of Marsaglia and Tsang's Ziggurat method for generating random deviates can lead to poor distributions. Changing the uniform random number generator used in its implementation fixes this issue.